1. Field of the Invention
The invention relates to a process for the preparation of cement clinker and special binder qualities with high .alpha.'-belite contents and a high final strength from metallurgical slags, in which liquid slags from reduction processes (acid slags) and steelworks processes (basic slags), such as blast furnace and converter slag, are mixed together and, if necessary, mixed with lime.
2. Background Information
In addition to conventional Portland cements, slag cements in the narrower sense have already assumed great importance, depending on their concrete properties. Slag cements in the narrower sense are finely ground mixtures of gypsum, clinker and blast furnace slag. Like Portland cements, they contain, as a rule, some gypsum to regulate the binding time, and when the mixture contains less than about 30% blast furnace slag, it is known as iron Portland cement and as blast furnace cement only when there is a higher proportion of blast furnace slag.
On the recovery of iron in the blast furnace or by other metallurgical reduction processes, iron ores, for the most part oxygen compounds of iron, mixed with solid fuels such as coke are heated to temperatures of about 1600.degree. C. When a blast furnace is used for the reduction of iron ores, it is not, as a rule, possible to dispense with coke and thus high-grade carbon-containing material. In other known reduction processes cheaper charge coal may be used, at least in part. The higher sulphur content of the charge coal which is often observed in these cases may, through appropriate management of the slag, be brought at least for the most part into the slag. The oxygen of iron oxide combines in such a reduction process with the carbon of the coke or the charge coal to form CO.sub.2 and CO and an iron bath which is separated in liquid form is produced.
Since, however, iron ores contain not only oxygen compounds of iron, but also a series of impurities, the so-called gangue, these impurities, which are predominantly of an argillaceous nature and consist of silicic acid and alumina, must be separated. The melting temperatures of such mixtures of silicic acid and alumina are, as a rule, slightly higher than the melting point of iron, and in order to improve separability an appropriate lowering of the melting point of these mixtures of silicic acid and alumina must therefore already be achieved in the reduction process. For this purpose lime is added to the charge in order to obtain a relatively readily melting mixture of lime, silicic acid and alumina which is in the form of a slag melt and floats on the heavier iron melt. The slag, like the iron, can subsequently by tapped off from time to time.
The lime charge must naturally be selected with the metallurgical parameters taken into account and cannot be adapted solely to the required composition of the slag. Blast furnace slag, however, is a substance very closely related to Portland cement clinker, since Portland cement clinker also has the main components silicic acid and alumina. Portland cement is for the most part richer in lime than blast furnace slag, while admittedly if an attempt was made to add to the slag so much lime that the composition of Portland cement is reached, a distinct rise in the melting point of the mixtures would be obtained, which would greatly hamper the required slag reactions in the blast furnace or would lead to alkaline circulations and charges.
The composition of blast furnace slag concretely obtainable in each case thus depends on the composition of the gangue of the ores and naturally also on the subsidiary components of the lime added. In the blast furnace process, for example, limestones are used which have high dolomite fractions, and thus introduce into the slag magnesium as well as calcium.
Thus, taking into account the circumstance that a relatively low melting point is aimed at, the slag cannot be managed randomly in order to arrive at a suitable clinker which gives a high-grade cement.
In addition to blast furnace slags, converter slags which, in comparison with blast furnace slags, have a much higher iron oxide fraction and metallic iron fraction and in principle have poorer hydraulic properties after the grinding of a corresponding slag clinker also occur in metallurgical processes. Water granulation of converter slag is extremely dangerous on account of the formation of detonating gas (Fe+H.sub.2 O.fwdarw.Feo+H.sub.2). Methods are also known of greatly accelerating the curing behaviour of iron Portland cement or blast furnace cement by specific additives such as quicklime, hydrated lime or gypsum, for example. A sulphate acceleration of blast furnace slag, in particular by supersulphated cement, is characterized by a particularly rapid curing rate.
The use of slag from a LD converter together with blast furnace slag for the purpose of common sintering is known from DE-PS 16 46 685. It was already proposed in DE-OS 26 11 889 to subject smelter waste together with lime to an oxidizing smelting, after which the finished melt may be quenched to a granulate, and, finally, the granulate can be ground to cement with the addition of gypsum. This latter invention in particular, from which the present invention proceeds, here already makes use of the latent heat of the melting, since the smelter waste can be used in a suitable mixture in molten form, for example as blast furnace slag and steelworks slag, together with smelter waste sludges and other additives.